Observational and National Centers for Environmental Prediction (NCEP) analysis data are used to diagnose the characteristics of synoptic circulations, sources of water vapor, and mesoscale convective systems (MCS) related to the formation of heavy rainfall on July 21, 2012. Results indicate that the locations and intensities of upper- to lower-level synoptic circulations and mid- to low-latitude systems are favorable for the heavy rainfall formation over North China that occurred under typical weather patterns with high pressure to the east and low pressure to the west. Synoptic systems such as vortices, shear lines, troughs, cold fronts, and low-level jets affect the formation and intensity of heavy rainfall. Tropical cyclones in the Bay of Bengal and the western Pacific Ocean provide favorable conditions for the transportation of water vapor from the ocean to the mainland. The sources of water vapor to North China are the Bay of Bengal, the Bohai Sea, and the Yellow Sea. Water vapor in the lower troposphere is mainly transported from the latter two sources, and water vapor in the middle troposphere is mainly transported from the former. The entire heavy rainfall process involves two stages. During the first stage, the triggering of MCS is associated with cold air invasion, terrain, and easterly wind in the prefrontal area. During the second stage, the organization and intensification of MCS are forced by the cold front. Under favorable synoptic circulations, the frequent development of stable and persistent MCS is important for the formation of the heavy rainfall event examined in this study.